Joint implants have been designed to replace articular surfaces of many human and animal joints. These implants include replacements for hip, knee, elbow, shoulder, ankle, and other joints. Prosthetic joint implants generally must both replace the bearing surface of the joint for motion and provide some method of fixing the implant to the bone.
Conventional methods of fixing the implant to the bone has been accomplished by two primary methods: bone cement (e.g., PMMA) or bone ingrowth through porous surfaces (biologic fixation). Bone cement is mixed in the operating room and injected into the preparation sites for the implants. The strength of a joint implant's fixation using bone cement is at a maximum shortly after implantation (usually after around 24 hours). Over time, the cement can fatigue and break down, leading to loosening and wear of the implants.
Biologic fixation for prostheses eliminates the need for cement and possibly avoids the long term complications associated with cement fixation. Conventional biologic fixation methods include fixation through a porous surface created by layers of sintered beads, plasma sprayed surface, titanium wire, and other micro and macro surfaces providing for a mechanical lock with bone. Bone ingrowth within the first 10-12 weeks after surgery essentially determines whether the implant will be stable long term. In order to be successful, the porous surfaces or macro surfaces must be well designed for bone attachment and the implant must be very stable during this early post-operative time period. If there is too much motion between the implant and the bone, a fibrous tissue interface will occur rather than bone ingrowth. Should that happen, the long term success will be in jeopardy as loosening due to pain and micromotion can create a need for revision.
For stability to be consistently achieved during the preparation of the bones for the implant insertion, very precise instruments and very consistent surgical techniques are required. In many cases this cannot consistently be achieved due to many variables including surgeon error, worn instruments, and patient issues. In some cases, discovery of a problem may come after insertion of the implant. For example, during surgery, after the bones have been prepared for biologic fixation, trial implants placed and evaluated, and the final implants selected and inserted into their corresponding preparations, the surgeon may discover that the patient's joint is not suitable for biologic fixation.
However, addressing these problems usually requires removing the implant and/or attempting further preparation to the bony beds. Different bone preparation techniques are required for cement fixation, and thus switching from one method to the other mid-surgery typically requires removing the implant and essentially starting over from the beginning. Further, conventional joint implants are subject to post-operative loosening due to, for example, inadequate stability due to initial patient activity. There thus remains a need for an improved joint implant that can address these and other disadvantages in the prior art joint implants and implantation methods.